Populations of blue crabs, striped bass and anchovies are growing in Chesapeake Bay amid oxygen levels that are among the highest in three decades in the estuary.
Samples taken in late June revealed that the area of water at the bottom of the bay that contains little or no oxygen has shrunk to about 40 percent of its long-term average, at less than half a cubic mile of volume, according to the state Department of Natural Resources.
That was the second-smallest volume for late June since scientists started closely monitoring the data in 1985, according to the Baltimore Sun.
"We're kind of dialing things back about 30 years," William Dennison, vice president for science applications at the University of Maryland Center for Environmental Science, told the newspaper.
Although the abundance of seafood or underwater grasses are visible signs of the bay's rebounding health, oxygen levels are an unseen yet imperative part of its ecosystem. "Dead zones" devoid of oxygen kill fish, crustaceans, bivalves and even phytoplankton.
Oxygen levels are "a truly integrative kind of measure," Dennison said. "When that's getting better, that's really the ultimate good news story."
The air and underwater plants dissolve oxygen in the water, but the gas becomes depleted in a process seen in waterways across the country and globe: Nutrients from farm runoff and sewage leaks fertilize algae blooms, and when the algae blooms die, their decomposition strips the vital element from the water.
Chesapeake Bay oxygen levels are lowest in an area that stretches along the bottom of the waterway from the Bay Bridge south to the mouths of the Patuxent and Potomac rivers.
"Fish can swim away from an area of no oxygen, but clams and oysters, if they are exposed to areas of no oxygen, can't get away, so they die," said Bruce Michael, director of resource assessment for the natural resources department.
The biggest factors influencing the dead zones are the flow of the Susquehanna River, which carries half of the bay's fresh water and much of its nutrients, and other efforts to limit nutrient runoff from land and leaks from sewage plants. Susquehanna flow is expected to be average or below average this summer as efforts to limit farm fertilizer use and modernize sewage treatment continue to ramp up.
Algae blooms have been a problem in waterways from Wisconsin to Florida.
Sen. Marco Rubio, R-Fla., recently joined Norcross Marine Products CEO Greg Lentine and members of the South Florida Water Management District for a boat ride along the St. Lucie River to see the devastation the algae bloom caused.
The senator urged President Obama on Thursday to approve Florida Gov. Rick Scott’s request for a federal emergency declaration in response to the algal blooms stemming from Lake Okeechobee discharges.